To understand the importance of the invention discussed herein, it will be helpful to have a background in the following areas that play major roles in microwave ovens and their use. The first area of discussion is the nature of microwave energy, its application to cooking, and its effects on liquids. The second area of concern is the makeup of foods and their reaction to microwave energy. The third area to be discussed is the positioning of the food to be cooked within the microwave oven cavity to achieve optimum results.
Microwaves are a form of electrical energy that resemble radio waves. These waves are disseminated within a microwave oven cavity in a random manner. They bounce off the six sides that make up the cavity. The energy is both radiated onto, and reflected off the walls. Liquids react to microwaves more than solids for the molecules are able to move, and therefore react, due to the amorphous nature of liquids.
Foods consist mostly of liquids or moisture. On a cellular level, organic foods consist of approximately 80% water. The reason for heating foods is to soften the cell walls so that the nutrients are more readily available. When the cell walls are uncooked, there is more need for physical means to break down the cellular content of the food via chewing. Conversely, when a food is over-cooked, the cell walls are either burned or they lose their water. Loss of water also makes the food unpalatable. Also, the nutrients are often lost with the water vapor. In microwave cooking the water absorbs the energy and heats the solid portion by conduction. We are concerned with even equal heating of the food so that all the food attains the same degree of cell softening in the same period of time. This provides optimum availability of nutrients.
The best way to secure these benefits in the microwave oven is to suspend the food in a vertical position so that it does not rest on or against any of the six walls of the oven cavity. In the example we will use, the relative values will remain constant throughout. We will use a package of frozen peas that is a solid rectangle with the dimensions of 4".times.3".times.1". Vertical positioning calls for the 4" side to be vertical. In the horizontal attitude, the 1" side is vertical. The internal dimensions of the microwave oven cavity are 12" high, by 14" wide, by 10" deep. These spacial relationships are depicted in FIG. 1A. The relative positions have a definite effect on the evenness of cooking. If one suspends the block of vegetables vertically, 100% of the surface area of the block is available for energy penetration. If the block is placed on the floor of the oven (the most general way to cook in the microwave oven), depending on the side placed on the floor of the oven, from 7.9% to 31.5% of the total surface area of the block of frozen peas is lost to "equal energy penetration" due to its positioning (FIG. 1B).
However, cooking such foods in the vertical position presents a problem of stability. One solution to this problem is to provide a container with an extending supporting base to hold the container in a vertical position. This solution is inadequate because a base would interfere with the container when it is in a horizontal position, making it difficult to ship the container in a case with other containers and also making it undesirable for use as a serving dish.
It is readily apparent that if the block is suspended in the vertical position, discussed earlier, in the center of the oven cavity it will receive energy more evenly than any other attitude. This reduces by one-half the distance the microwave energy has to travel to reach any molecule. Returning to our example of the frozen peas (a block: 4".times.3".times.1"), the shortest distance that would have to be traveled would be 1/2". More even cooking is obvious because one-half of the energy enters the food from both front and back simultaneously, rather than from only one side, all at once. When the block is horizontal (one side of the cube resting on the floor of the oven), it receives energy from the top, 2 edges, 2 ends, but not the bottom: it is 17% less effective, when one considers the exposed surface area (only 5/6th of the surface is exposed to direct radiation when compared with the vertical position).
Positioning, therefore, can make a significant difference in the evenness of the cooking within a microwave oven. It is this positioning, and its proven importance, that is addressed with the subject high efficiency cooking container for microwave ovens.
The present invention provides an inexpensive microwave cooking container for cooking food in a vertical position in a microwave oven which significantly increases even cooking throughout the food. The container can be thermo formed from a thin plastic material and so has nomininal rigidity. Consequently, a relatively small amount of material is needed to construct the container; it can be disposed of after being used once. The weight of the food provides stability to the container in a vertical position. The container is unstable in a vertical position when empty, but it is stable when full.
The weight of the food shifts the center of gravity of the filled container to within the container itself (that is, above a portion of a sidewall upon which the container is resting in its vertical position). If the container were initially stable without the food, the weight of the food could shift the center of gravity of the container and the food far beyond the container, thereby causing it to tip over. The container is designed so that its center of gravity is outside of the container when it is empty. The weight of the food shifts the center of gravity centrally within the container, thus providing stability to both the container and the food therein.
Because the container has no supporting base extensions, it can be readily positioned horizontally after it is removed from the oven, and, accordingly, used as a serving tray for the cooked food. Furthermore, because of the absence of such supporting extensions, a plurality of the containers can be packaged together in a shipping case with minimal interference between containers. Also, because the container is not a closed structure, but rather has an opened top surface, it provides easy packaging of the food at the factory and easy access to the food by the consumer at home.
Food can be stored in the container prior to cooking. Thus, the container provides a vehicle for storing food, cooking food, and serving food after it has been cooked. All of these operations are performed without transferring the food from the container.
The container provided is inexpensive to manufacture, requires a relatively small amount of material to construct, can be disposed of after use, and can be used for storing food, cooking food, and serving food after it has been cooked. Because food can be cooked vertically within the container and thus be centered in the oven to absorb an optimum amount of both direct and reflected microwave radiation, the food is more evenly cooked throughout its mass, than were it cooked in any other attitude within the oven category.